Abstract: Immediate hypersensitivity reactions are dependent on IgE-mediated activation of basophils and mast cells. There are many elements that regulate expression of the response that distribute up and down the cascade of events from the generation of IgE to the tissue end-organ response and the positive and negative feedback loops that link the various elements together. One of the early necessary elements is related to the intrinsic ability of the basophil and mast cell to respond to aggregation of the IgE receptor. It is now apparent that for human basophils, this intrinsic responsiveness primarily relates the expression of one of the early receptor associated kinases, SYK. Several studies have established that SYK activity is likely a rate-limiting step in the IgE-dependent signal transduction cascade and expression levels determine its activity. Studies have also demonstrated that the very low expression levels of SYK in basophils are unique to this leukocyte. Recent studies demonstrate 4 regulatory pathways of SYK expression, two of which have recently been excluded for further consideration. Based on preliminary results, the application proposes to examine two new pathways in greater detail, one extrinsic and one intrinsic, modulation by SCF/Flt3 and basophil-specific differentiation transcription factors, respectively. Aim 1 of the proposal will refine pilot study observations on how Flt3L and SCF down-regulate SYK expression in maturing basophils. Aim 2 will validate preliminary results that suggest that c-MAF, KLF4 and/or ZNF608 regulate transcription of SYK at one of 3 regions of the SYK gene. Aim 3 will extend the results of aim 2 to the human mast cell, testing whether regulation of SYK by the transcriptional control discovered for basophils operates in mast cells.